The Long-Term Evolution (LTE) of the Universal Terrestrial Radio Access Network (UTRAN), also denoted E-UTRAN, as standardized in Rel-8 of the 3rd Generation Partnership Project (3GPP) specifications supports transmission bandwidths up to 20 MHz. In the downlink, LTE uses conventional Orthogonal Frequency Division Multiplexing (OFDM) as the transmission scheme. OFDM provides benefits, e.g. it is robust to time dispersion, but has also some drawbacks, most notably a relatively high peak-to-average power ratio (PAR) of the transmitted signal.
Power amplifiers have to be designed to meet peak transmission power requirements while still meeting network requirements regarding the average output power (for example, determining the achievable data rate and coverage). The difference between the peak power and the average power determines the so-called amplifier back-off and is thus a measure on how much the power amplifier needs to be “over dimensioned” (or, equivalently, how much is lost in coverage when using the same amplifier but a lower-performance transmission scheme).
A high PAR implies a larger power back-off in the power amplifier, that is, the power amplifier cannot be used to its full extent. The Cubic Metric (CM) is another, generally more accurate metric, that can be used to represent the amount of back-off required in the power amplifier. In the following, the term “power amplifier metric” (denoting, e.g., PAR, CM, or any other appropriate measure) is used which shall be generally understood as a measure representing the impact of the difference or ratio between the peak power and the average power on the power amplifier design.
In the uplink, a high power amplifier metric can lead to reduced coverage, higher battery consumption, and/or more expensive implementation. Therefore, for the uplink, LTE has adopted a single-carrier transmission scheme with low power amplifier metric known as DFT (Discrete Fourier Transform)-spread OFDM (DFTS-OFDM) or DFT-precoded OFDM (sometimes also referred to as Single-Carrier Frequency Division Multiple Access, or SC-FDMA). SC-FDMA exhibits a significantly lower PAR than OFDM.
FIG. 1 is a schematic illustration of an example of an SC-FDMA transmitter stage 100 operable to transmit on a single carrier according to the LTE transmission scheme. In transmitter stage 100, DFT coder 105 is coupled to OFDM modulator 110 which in turn is coupled to power amplifier 120 through a cyclic-prefix insertion stage 115 operable to insert a cyclic prefix in the output from OFDM modulator 110 before the output is amplified by power amplifier 120 for transmission over carrier 125. As shown in FIG. 1, carrier 125 has a bandwidth of 20 MHz. Carrier 125 may be referred to as a frequency resource for the transmission of a set of data blocks. While in FIG. 1, carrier 125 is shown as having a 20 MHz bandwidth, other bandwidths are possible in the LTE transmission scheme, and the bandwidth may vary (e.g., depending on the number of symbols to be transmitted via carrier 125).
Modulation symbols 101, shown in FIG. 1 as M modulation symbols, are input to DFT coder 105 and the output of DFT coder 105 is mapped to selective inputs of OFDM modulator 110. Examples of OFDM modulators comprise an Inverse Fast Fourier Transform (IFFT). The output of OFDM modulator 110 contains the data of modulation symbols 101 (“OFDM symbols”) and is amplified by power amplifier 120 for transmission over carrier 125.
The DFT size, for example the size of the DFT performed by DFT coder 105, determines the instantaneous bandwidth of the transmitted signal while the exact mapping of the DFT coder output to the input of the OFDM modulator 110 determines the position of the transmitted signal within the overall uplink transmission bandwidth. Similar to conventional OFDM, a cyclic prefix is inserted subsequent to OFDM modulation. The use of a cyclic prefix allows for straightforward application of low-complexity frequency-domain equalization at the receiver side.
In order to meet requirements for International Mobile Telecommunications-Advanced (IMT-Advanced), 3GPP has initiated work on LTE-Advanced. One aspect of LTE-Advanced is to develop support for bandwidths larger than 20 MHz. Another aspect is to assure backward compatibility with LTE Rel-8. Backward compatibility also includes spectrum compatibility. Thus, in one exemplary implementation, to allow for backwards compatibility with LTE Rel-8, an LTE-Advanced spectrum or carrier that is wider than 20 MHz may appear as a number of separate LTE carriers to an LTE Rel-8 terminal. Separate LTE carriers may be referred to as different frequency resources. Thus, each Rel-8 LTE carrier can be referred to as a single frequency resource.
For early LTE-Advanced deployments, it can be expected that there will be a smaller number of LTE-Advanced-capable terminals compared to many LTE legacy terminals. Therefore, it is desirable to enable the use of frequency resources such that legacy terminals can be scheduled in all parts of the available wideband LTE-Advanced bandwidth. The straightforward way to allow for such optimal backwards compatibility would be by means of frequency resource aggregation. Frequency resource aggregation implies that an LTE-Advanced terminal can receive and transmit on multiple frequency resources, where each frequency resource may have, or may be modified to have, the same structure as a Rel-8 LTE carrier.
An example of the aggregation of multiple frequency resources is illustrated in FIG. 2. Frequency resources 210 in FIG. 2 are all located next to each other so as to be contiguous. In the specific example of FIG. 2, each frequency resource has a bandwidth of 20 MHz. Together, the five frequency resources 210 shown in FIG. 2 aggregate to an aggregated bandwidth of 100 MHz. The frequency resource aggregation shown in FIG. 2 requires that the operator has access to a contiguous spectrum allocation which can be divided to achieve the number of aggregated frequency resources. While in the drawings frequency resources are shown having a bandwidth of 20 MHz, this is for purpose of illustrating a backwards compatible spectrum allocation. Generally, individual frequency resources may have any bandwidth depending upon the number of included subcarriers.
To provide additional spectrum flexibility, LTE-Advanced may also support aggregation of non-contiguous spectrum fragments, which may be referred to as spectrum aggregation, an example of which is illustrated in FIG. 3. In the particular example of FIG. 3, five frequency resources 210 are spectrum aggregated to provide an aggregated bandwidth of 100 MHz. One or more frequency resources 210 are separated by spectrum gaps 320 which separate the one or more frequency resources 210 such that those frequency resources 210 separated by spectrum gaps 320 are not contiguous. Spectrum aggregation allows for the flexible addition of spectra for transmission. For example, an operator may bring into use different spectrum fragments over time depending upon availability for use by the operator.
The DFTS-OFDM property of a relatively low power amplification metric should be maintained as much as possible when extending the transmission bandwidth across multiple frequency resources, as for example, part of achieving or adding spectra to an LTE-Advanced system (e.g., having a spectrum allocation such as that shown in FIG. 3). To achieve a system operable to implement LTE-Advanced by extending the transmission bandwidth across multiple frequency resources, the structure of transmitter stage 100 of FIG. 1 may be generalized to transmit on one or more distinct frequency resources as shown in FIG. 4.
FIG. 4 is a schematic illustration of an example of such a generalized transmitter stage 400 operable to be compliant with LTE-Advanced by transmitting on multiple frequency resources. In transmitter stage 400, DFT coder 105 is coupled to OFDM modulator 110 which in turn is coupled to power amplifier 120 through a cyclic-prefix insertion stage 115 operable to insert a cyclic prefix in the output from OFDM modulator 110 before the output is amplified by power amplifier 120 for transmission over different frequency resources 410a, 410b. 
As shown in FIG. 4, transmitter stage 400 may be operable to receive modulation symbols 401 for transmission on frequency resources 410a, 410b substantially simultaneously. As can been seen from FIG. 4, frequency resources 410a and 410b are separated by spectrum gap 420 and are hence non-contiguous. As also shown in FIG. 4, each frequency resource 410 has a bandwidth of 20 MHz, thus the spectrum aggregation of the two frequency resources yields a total bandwidth of 40 MHz.
In the system of FIG. 4, DFT coder 105 and OFDM modulator 110 are scaled to match the larger bandwidth. The output of DFT coder 105 is connected to the input of OFDM modulator 110. Because the two frequency resources 410 are not contiguous in frequency, zeros will be input to OFDM modulator 110 to allow for gap 420. In one embodiment of a possible future extension, the control signaling on the Physical Uplink Control Channel (PUCCH) may be located at each of the band edges of the LTE uplink, that is, for example, at the band edges of each frequency resource.
The structure shown in FIG. 4 is sometimes referred to as Clustered DFTS-OFDM (CL-DFTS-OFDM), where the term clustered refers to the fact that the frequency resources are not necessarily contiguous in frequency but located close to each other. The power amplifier metric of the generated signal is higher than that of conventional DFTS-OFDM, as shown, for example, in FIG. 1, but still low compared to OFDM and increases with the number of clusters.